Given how the Hobbywing rotor barely fits on there while other rotors need the gap, the part that moves will likely need to be the part the rotor directly rests on, rather than having a separate stopper. More parts means more interfaces that have tolerances. Depending on the design there could be additional tolerances related to friction, or fastener tension distorting the material etc. In short, more parts and more complexity means more chances for tolerance issues to pop up.